I understand not every decade was a clean break, but each decade has fairly distinct defining properties. It feels like most of the 21st century has been a single run on with smaller changes. I know sometimes the definitions don’t come into focus until later, but I’ve been around since the 80s and I can distinctly remember the changes between the 80s, 90s and 00s as they were happening.
If you live in the US, George W. Bush went to war in Afghanistan in October 2001 and war in Iraq in February 2003 and Donald Trump arranged the surrender/withdrawal from Afghanistan on Feb 29th, 2020, ending the war exactly as the COVID-19 pandemic was ramping up. Then in January 2021 there was a coup attempt just as the first COVID-19 vaccines were becoming available for essential workers outside healthcare.